Electron accelerator for producing roentgen-ray pencils deflectable in space



July 2, 1957 R. WIDEROE 2,798,177

ELECTRON ACCELERATOR FOR PRODUCING ROENTGEN-RAY PENCILS DEFLECTABLE IN ACE Filed July 25, 1952 2 SheetsSheet 1 IN VENTOR BY Wt, WW9 ATTORNEYS y 2, 1957 7 R. WIDEROE 2,798,177

ELECTRON ACCELERATOR FOR PRODUCING ROENTGEN-RA PENCILS DEFLECTABLE IN SPACE Filed July 25, 1952 2 Sheets-Sheet 2 INVENTOR i BY M J e 2, 1 ATTORNEYS United States Patent ELECTRON ACCELERATOR FOR PRODUCING ROENTGEN-RAY PENCILS DEFLECTABLE 1N SPACE Rolf Wideriie, Ennetbarlen, Switzerland, assignor to Aktiengesellschaft Brown, Boveri & Cie, Baden, Switzerland, a joint-stock company The present invention relates to electron accelerators as for example those commonly known as magnetic induction accelerators or ray transformers wherein electrons are accelerated round and round an orbit located inside of an evacuated tube under the combined actions of time varying magnetic induction and guiding fields. At the end of the acceleration phase, the electron stream is caused to be diverted from its orbit and impinge upon a target or anti-cathode for the production of very hard Roentgen rays which are used in nuclear physics or in the field of medicine for treatment of cancer, etc. A typical construction is shown in my United States Patent No. 2,586,494 issued February 19, 1952.

The Roentgen rays developed in these accelerators are produced in very narrow pencils and the angular spread of the pencil in a megavolt unit for example amounts to no more than about 7 degrees which has been found too small for many applications. A somewhat uniformly intense pencil above a certain space angle can be obtained for example by bringing an absorbent with a suitable variable absorption above the space angle into the pencil. However it will be evident that this method can be used only at the sacrifice of a great loss in ray intensity and an undesirable increase in the scatter radiation and density of the secondary electrons.

Not all applications make it necessary for the ray pencil active in the desired space angle to have in every moment an approximately uniform intensity distribution above the space angle. On the contrary it is sufficient for the integrals of the intensity extending over the operating time to have approximately the same value in all of the space angle elements. This result can be obtained, in accordance with this invention, by moving the axis of the ray pencil relative to the whole apparatus during the operating time within the space angle desired to be irradiated. At this point, it should be noted that upon the occurrence of energy rich electrons on an anti-cathode, the direction of the Roentgen-ray pencil produced is the same as that of the electrons. While the ray axis could theoretically be shifted by a movement in rotation of the whole device about its axis, such a method is wholly impractical in view of the great masses involved in the magnetic structure necessary to produce the high intensity magnetic fields. Consequently the present invention is predicated upon the novel and yet practical concept of shifting the ray pencil relative to the body of the accelerator by varying the direction of impact of the accelerated stream of electrons on the anti-cathode through a selected angle.

The accompanying drawings, all schematic in character, illustrate several embodiments of this inventive concept. Moreover, in order to simplify matters, the drawings include only those elements of the electron accelerator essential to a clear understanding of this invention which is obviously only an improvement upon the already known general principles of construction for this type ice of apparatus which can be obtained by reference to my aforesaid Patent No. 2,586,494.

Fig. l is a view in horizontal section through the tube element of the accelerator showing an embodiment wherein the shifting of the axi of the ray pencil in the plane of the tube is accomplished by displacement of the anti-cathode element itself;

Figs. 2 and 4 are also views in horizontal section through the tube element illustrating two different arrangements for coils producing magnetic fields which effect the same result without moving the anti-cathode.

Figs. 3 and 5 are explanatory diagrams related to Figs. 2 and 4, respectively;

Fig. 6 is a view in perspective of still a third possible arrangement of ray deflecting coils but wherein deflection of the ray pencil occurs in a plane perpendicular to the plane of the tube; and

Figs. 7 and 8 are views in perspective illustrating different arrangements of parallel spaced annular electrodes by which deflection of the ray pencil in a plane perpendicular to the plane of the tube is eifected by electrostatic field forces produced between the electrodes.

With reference now in particular to Fig. l, the annular tube element in which acceleration of the electrons takes place is shown in section at 10. The anti-cathode element 11 is disposed within the tube and offset radially from the orbit on which the electron stream is accelerated after being emitted from an electron source such as a cathode which is also usually located within the tube. Neither the cathode nor acceleration orbit have however been included. Near the end of the acceleration phase the electron stream is caused to be shifted spirally from its orbit in the direction of the anti-cathode 11 by any known means such for example that shown in United States Patent No. 2,103,303 issued December 28, 1937 in the name of Max Steenbeck. The accelerated electron stream impinges upon the anti-cathode element 11 and the electron path just prior to impingement is denoted by the letter 8. The pencil of Roentgen-rays produced by such impingement leave the anti-cathode 11 in the direction R. In order to effect a change in the angle of the ray pencil it will be seen that anti-cathode 11 is secured upon an arm 12 arranged for rotation about an axis 13 which coincides at least approximately with the axis of the electron accelerator. Arm 12 passes through a vacuum tight bellows 14 which enables the arm to move and thus also anti-cathode 11. If the arm 12 and anti-cathode 11 are displaced rotationally clockwise to the position 11' shown in dotted lines, then the Roentgen days will depart in the direction of the dotted line arrow R. In this way, the maximum of the Roentgen ray pencil will have thus been displaced through the angle 0:. Suitable means acting in the proper timed relation with movement of the electron stream are of course provided to shift the position of the anti-cathode 11 at the proper time so that the ray pencil will be swept through the angle a.

If it is desirable, for Roentgenoptical reasons, to effect a displacement of the ray pencil along a straight line, i. e. rectilinearly then the center of rotation 13 can be shifted simultaneously with the rotation in the direction toward the middle position of the anti-cathode.

A second possibility for effecting a shift in the my direction by actual movement of the anti-cathode would be to secure the latter immovably to and within the tube and then rotate the whole tube about an axis generally coincident with that of the accelerator. As with the Fig. 1 embodiment, the center of rotation of the acceleration tube can be shifted simultaneously with its rotation if it be desired to shift the ray along a straight line.

Figs. 2 and 3 illustrate an embodiment for the invention wherein the position of the anti-cathode remainsfixed and deflection of the ray pencil is accomplished by electromagnetic forces. In this embodiment the anti-cathode 111 is fixed in space inside of the acceleration tube 11%) and the electron path tothe anti-cathode is again de noted'by {3. The tube 110 is' surrounded by a coil 15 placed at the inner and outer sides of the tube and wound in such direction that when traversed by direct current in the direction denoted by the arrows, magnetic'fields will be produced in the tube 110 which are perpendicular to the plane of the tube and hence also perpendicular to the electron orbit therein and which fields have opposite di-f rections on opposite sides of a diameter of the tube" laid through the fixed anti-cathode 1 11. Coil 1 5 is caused to be energized with direct current at theclo's'e of the ac: celeration phase by suitable control-switching meansial ready knownand therefore not illustrated. One'generally suitable arrangement for energizing the coilat the proper time is shown inFig. 5 of United States Patent No. 2,394,070 issued February 5, 1946 iii the'name of Donald W. Kerst; V

The magnitude and direction of the force eXe'rted by these fields on the electrons can be seen from the width and position of the hatched surfaces shown in Fig.- 3. Thus there are exerted on the electrons radial forces which cause the path of the electrons to be deformed in the plane of the acceleration tube from shape 13 to 5'. Accordingly the anti-cathode 111 will be bombarded from a different direction and the maximum'of the pencil of rays is caused to be shifted through angle oi from R to'R.

The arrangement of the coil 1 5 in Fig. 2 is such that the magnetic field is substantially uniform around the entire circumference of the tube. Because of this, large deflecting angles or cannot be obtained for the reason that the electrons at places 16 and 17 lying on opposite ends of a diameter normal to that containing anti-cathode 111 would get too far away from the acceleration orbit and might even be lost by impingement upon the walls of the tube. If large deflection angles on are: required, the embodiment shown in Figs. 4 and 5 should be used. This arrangement is similar to that shown in Figs. 2 arid 3 er} cept that the coil 18 surrounding tube 210 containing fixed: anti-cathode 211' isprovided with an" extra number of turns 18' in the vicinity of the anti-cathode than elsewhere around the tube. The corresponding intensified magnetic fields of opposite direction produced at opposite sides of the anti-cathode 211 thus bring" about a correspondingly increased deformation of; the electron path at such locations as' shown clearly in Fig; 5 with the result that the electrons arriveatthe anti-cathode 211 along a greatly altered path 13 which brings about a greater deflection angle a for the maximum of the ray pencil.

Fig. 6 illustrates an embodiment which provides a deflection of the maximum of the ray pencil in a plane perpendicular to the plane of the tube 310. The deflection force is derived magnetically as in Figs. 4 5 by acoil-19 which surrounds the tube and is so wound that when traversed by direct current at the end of the acceleration phase of the electron stream, magnetic" fields will be produced in tube 310 which are parallel to the plane of the tube and hence also parallel to the electron orbit therein and which have opposite directions on. opposite sides of a diameter through the tube 310' laid through the fixed anti-cathode 311. That is the field produced by one half the coil 19 is directed toward such diameter and that produced by the other half of this coil is directed away from such diameter. As in' the Fig.4 embodiment, both halves of coil 19 have extra turns 19' in the vicinity of anti-cathode 311 to provide increased field strength at these places' which gives a higher deflection angle for the ray pencil. The operation of the Fig. 6 embodiment is similar to that of Figs. 2-5 except that deformation of the path of the election stream from path B to path 3 will take place in a plane perpendicular to the plane of 4 tube 310. Consequently deflection of the maximum of the ray pencil from anti-cathode 311 will be through angle 0 in a plane perpendicular to the plane of the tube.

In the embodiments described, deflection of the ray pencil is effected by means of electromagnetic forces. The same result can be obtained through use of electrostatic forces. Embodiments of the latter are illustrated in Figs. 7 and 8.

In Fig. 7, the tube is designated by numeral 410 and the anti-cathode fixed therein by 411. Above and below the tube are a pair of parallel spaced angular electrode plates interrupted along a diameter of the tube 410 laid through anti-cathode 411. Thus the top plate consists of two semi-circular sections 21, 21a and the lower plate likewise consists of two semi circular" sections 22, 22a. These four sections are cross connected so that the electrostatic fields produced respectively by sections 21-22 and 21a'22a at opposite sides of the diameter through anti-cathode-41'1' in a plane perpendicular to the plane of tube 410 and to the electron orbit will be inopposite directions. That is to'say electrode sections 21 and 22'a are connected to one and the same side of the source of voltage 23 While electrode sections 21a and 22 are connected to the other side of that voltage source. As in the other described arrangements, the voltage is applied to the electrode plates at the close of the accelerating phase in such manner that the degree of influenceon the electron path varies during: the operating time and hence causes the e'lectr'o'npath to be deformed from path ,3 to path ,8 and a correspondingdeflection of the maximum of the ray pencil through vertical angle 1/1 from direction R to R;

The arrangement shown in Fig.- 8 is analago'u's to that shown in Fig. 5 in-tha't greater angles of deflection of the ray pencil are obtained by increased field strength in the vicinity of the anti-cathode. Consequently Fig. 8 represents a modification of Fig. 7 where in additionto the cross connected electrode plate sections 24', 25a and 24a, 25 and energizing voltage source 26 therefor there are provided additional arc'uat'e electrode sections 27, 27a and 28, 28a in the vicinity of anti-cathode 511. Upper electrode section" 27 at one side of the diameter through tube 510 laid on anti oathode- 511 is connected r with lower section 28a at the other side of such diameter to one side of voltage source 29 at the proper instant, and similarly the other upper electrode section 27a is connected with'the" other lower section 28 to the other side of source 29'. Source 29' is; at higher voltage than source 26 and hence the electrostatic field" will be stronger in the vicinity of anti-cathode 511 than at any other location around the" circumference of the tube. However it should be noted that Whereas in Fig. 5 defiection of the ray pencil takes place in a plane parallel 1 to that of tube, the ray deflection in Fig; 8 takes place in a plane perpendicular to the plane of the tube.

It is of course also possible to arrange the electrodes so as to establish electric fields in the tube whose directions are parallel with the plane of the tube in which event deflection of the ray pencil will also be in the plane of the tube. V v

In closing it' is desired to emphasize that While the invention has been described in its relation to electron accelerators ofthe ma netic induction type, it is also possible to apply the principles disclosed herein to other typesof electron accelerators wherein acceleration of the electrons takes place on a" closed path without departing from the spirit and'scope of the'invention as defined in the appended claims.

I claim:

1. An electron accelerator including an evacuated tube providing an orbital path for accelerating. a stream of electrons, an anti-cathode in said tube, and external- 1y controllable current traversed coilmeans located adjacent said tube and anti-cathode on opposite sides of said orbital path and on opposite sides of a diameter of said tube laid through said anti-cathode, said coil means on said opposite sides of said tube diameter producing magnetic fields respectively of opposite direction relative to said orbit on opposite sides of said diameter of said tube laid through said anti-cathode and which fields effect a variation in the angle of impingement of said electron stream upon said anti-cathode, whereby the pencil of Roentgen rays produced by said impingement is likewise shifted through an angle to effect radiation over a relatively large angle.

2. An electron accelerator as defined in claim 1 and wherein said coil means include means for producing a magnetic field adjacent said anti-cathode greater than the magnetic field elsewhere in said tube.

3. An electron accelerator as defined in claim 1 wherein the magnetic fields produced respectively by said coil means have a direction perpendicular to the plane of said tube.

4. An electron accelerator as defined in claim 1 Where'- in the magnetic fields produced respectively by said coil means have a direction parallel to the plane of said tube.

References Cited in the file of this patent UNITED STATES PATENTS 1,790,073 Pohl Ian. 27, 1931 1,948,384 Lawrence Feb. 20, 1934 2,103,303 Steenbeck Dec. 28, 1937 2,193,602 Penny Mar. 12, 1940 2,297,305 Kerst Sept. 29, 1942 2,331,788 Baldwin Oct. 12, 1943 2,394,070 Kerst Feb. 5, 1946 2,640,924 McMillan June 2, 1953 FOREIGN PATENTS 591,005 Germany Jan. 15, 1934 

